In the mouse, and many other mammals, the initial stage of implantation involves adhesion of the embryo to the uterine liminal epithelium. Since blastocysts acquire adhesiveness at extrauterine sites, it is presumed that the acquisition of adhesivesness is intrinsically programmed by the embryonic genome and involves changes, at the molecular level, at the surface of the trophoblast. There are changes in the uterus as well that make it "receptive" to an implanting blastocyst. By using lectins as probes to analyze the trophoblast cell surface, it has been possible to detect qualitative changes in the glycocalyx that occurred coincident with the acquisition of adhesiveness by mouse blastocysts (Chavez and Enders, Devel. Biol. 97:267-276; Biol. Reprod. 26:545-552). The present research plan involves the use of the same battery of lectins as probes to detect possible changes at the surface of mouse uterine luminal epithelium throughout the uterus and also at specific sites of implantation during the stages before and during adhesion of the blastocysts. Additionally, an analysis of the trophoblast cell surface proteins during the peri-implantation period will be undertaken by the use of radiolabeling techniques followed by one and two-dimensional electrophoresis. The aim of these investigations is an understanding of the relationship between the early embryo and uterus during the implantation process, and the mechanisms by which the embryo attaches to and penetrates the uterine luminal epithelium.